1. Field of the Invention
The present invention relates to magnetic memory elements, and more specifically, to magnetic tunnel junction devices having structures that alter the effective coercivity of free layers in the devices, and compensate for the effect of magnetostatic coupling (Néel-type coupling) between ferromagnetic layers in the devices.
2. Brief Description of the Related Art
Various types of memory are used extensively in digital systems such as microprocessor-based systems, digital processing systems, and the like. Recently, magnetic random access memory (MRAM) devices have been developed for use as non-volatile random access memory.
MRAM devices are based on magnetic memory elements. An MRAM device frequently includes several magnetic memory elements arranged in a row and column array, with circuitry for accessing information stored in individual elements in the array.
Information is stored in each magnetic memory element in the form of a resistance state of the element. An electrical resistance state of a magnetic memory element changes based on the relative orientations of magnetic moments in adjacent ferromagnetic layered structures within the magnetic memory element. The orientation of the magnetic moment in one layered structure, referred to as a “pinned” structure, is fixed as a reference, while the magnetic moment orientation of another layered structure, referred to as a “free” structure, can be changed, using an externally-applied magnetic field, for example. Changing the relative orientation of the two layered structures results in a change in the resistive state of the magnetic memory element. The different resistive states are recognizable by electronic circuitry as bit-wise storage of data.
Typically, a magnetic memory element, such as a magnetic tunnel junction (MTJ) memory element, includes free and pinned ferromagnetic layers separated by a non-magnetic spacer. In an MTJ memory element, the spacer is referred to as a tunnel junction barrier layer. When the magnetic moments of the free layer and the pinned layer are aligned in the same direction, the orientation commonly is referred to as “parallel.” When the two layers have opposite magnetic moment alignment, the orientation is termed “antiparallel.”
In response to parallel and antiparallel magnetic states, magnetic elements present two different resistances to a current provided across the memory element. In magnetic memory elements, the current typically is provided in a direction perpendicular to the surfaces of the element layers. The tunnel junction barrier layer is sufficiently thin that, in the presence of adequate current, quantum-mechanical tunneling of charge carriers occurs across the barrier junction between the free and pinned ferromagnetic layers. The resistance of the device typically has minimum and maximum values corresponding respectively to parallel and antiparallel magnetization vector orientations of the free and pinned layers.
Current miniaturization trends require that magnetic memory elements be manufactured with layers that are very thin, some layers being in the range of only tens of angstroms in thickness. At these small dimensions, minute variations in surface morphology, roughness, and constituent grain size can impact the magnetic characteristics of each layer. Further, as the trend continues toward smaller magnetic memory elements with thinner layers, the input response (coercivity) of the elements increasingly is affected by magnetic interactions between the layers, such as interlayer magnetic coupling. Interlayer magnetic coupling can result, for example, in the magnetic vector of the free layer becoming preferentially oriented in a parallel or anti-parallel direction to the pinned layer. This preferential orientation can adversely destabilize the memory element, and introduce an undesirable voltage threshold (hysteresis) which must be overcome in order to switch the magnetic moment of the free layer from one orientation to another.
Various mechanisms by which interlayer magnetic coupling occurs include: direct ferromagnetic exchange coupling via pinholes, whereby ferromagnetic bridges are formed across the spacer layer; indirect oscillatory exchange coupling; and magnetostatic, or so-called Néel-type, coupling. As the size of magnetic memory elements becomes increasingly smaller, Néel-type coupling becomes a more dominant mechanism of interlayer magnetic coupling. In addition, as the shape of magnetic memory elements becomes more circular, Néel-type coupling becomes more dominant in interlayer magnetic coupling.
An idealized structure of a representative prior art MTJ memory element 2 is shown in FIG. 1. The memory element includes a free ferromagnetic layer 4 separated from a pinned ferromagnetic layer 6 by a tunnel barrier layer 8. Layers in the memory element structure, such as the free and pinned ferromagnetic layers, may be formed as a stack of several individual sub-layers. A seed layer 10 typically is provided below the free ferromagnetic layer 4. Pinned ferromagnetic layer 6 is maintained in a single stable magnetic polarity state by an antiferromagnetic exchange (pinning) layer 12. Other layers typically are included, such as a capping layer (not shown). As noted above, a disadvantage of the prior art MTJ memory element structure shown in FIG. 1 is that Néel-type coupling occurs between pinned layer 6 and free layer 4.
A plan view of prior art MTJ structure 2 is illustrated in FIG. 2. Accompanying the trend toward increasingly smaller element size is a trend from elements having an oblong shape to elements having a more circular shape. Stray fields in elements having an oblong shape can be compensated with synthetic anti-ferromagnets or additional ferromagnetic layers, as disclosed in U.S. Pat. No. 6,677,631 to Drewes, incorporated by reference above. Other methods of compensating for stray fields become necessary in elements having a more circular shape. As the shape approaches a circle, the ratio of length to width (L:W) approaches 1, and element 20 takes on the characteristics of a closed flux device. Moreover, a more circular device switches polarity states at lower field strengths. As the L:W ratio drops below about 1.2, Néel-type coupling between layers becomes more pronounced. As a result, in the absence of an applied external field, the magnetic moment of free layer 4 will tend to orient in the parallel direction under the Néel-type coupling influence of pinned layer 6. Consequently, the free layer 4 has different magnetic field strength switching thresholds when going from a parallel state to an anti-parallel state and vice versa. This produces a magnetic field offset which must be overcome to switch the free layer 4 from one orientation to the other.
An MTJ structure is desirable in which Néel-type coupling influencing the coercivity of free layers is reduced or controlled.